One- and two-qubit operations in Si-MOS quantum dots

Abstract

Quantum computers have potential to solve hard problems that even the most advanced supercomputers are not capable of, such as prime factoring, database search, and quantum simulation. Quantum bits, or qubits, made from silicon quantum dots is one scalable approach that can be used to realize a quantum computer. Even though the essential ingredients for this platform have been demonstrated, certain physical phenomena that can influence quantum bit behaviour, for example, the valley- and spin-orbit coupling, require further understanding. In this thesis, we investigate harnessing the effect of spin and valley orbit couplings in silicon quantum dots. We propose a mechanism for a significant enhancement in the electrically-driven spin rotation frequency for silicon quantum dot qubits in the presence of a step at a hetero-interface. We calculate single qubit gate times of 170 ns for a quantum dot confined at a silicon/silicon-dioxide interface.To understand the origin of spin-orbit coupling, we experimentally probe the g-factor for two quantum dot qubits as a function of magnetic field and find that they are dominated by spin-orbit interactions originating from the vector potential. By populating the double dot we probe the mixing of singlet and spin-polarized triplet states, which we conclude is dominated by momentum-term in spin-orbit interactions. Finally, we exploit the Stark shift to reduce its sensitivity to electric noise and observe an 80% increase in the coherence time T_2^*. We proved that when understood and controlled, the small but significant spin-orbit interaction, can be used as a powerful resource. While a variety of qubit systems have shown high fidelities at the one-qubit level, superconductor technologies have been the only solid-state qubits manufactured via standard lithographic techniques which have demonstrated two-qubit fidelities near the fault-tolerant threshold. Here in silicon quantum dots, we demonstrate the single-qubit randomized benchmarking with an average Clifford gate fidelity of 99.96% and two-qubit randomized benchmarking with an average Clifford gate fidelity of 94.7% and average Controlled-ROT fidelity of 98.0%.The enhanced understanding of valley-orbit and spin-orbit coupling, together with demonstrated high fidelity single- and two-qubit gates provides opportunities that are promising for the scalability of spin qubit systems